Serializer not generated in test fixtures (testFixtures)

[jonnyandrew]

Describe the bug

I have enabled test fixures in my Android project, which is working.

However, when I try to create a class annotated with @Serializable in the testFixtures source set, the serializer doesn't seem to be available in my test:

kotlinx.serialization.SerializationException: Serializer for class 'MyClass' is not found.
Please ensure that class is marked as '@Serializable' and that the serialization compiler plugin is applied.

	at kotlinx.serialization.internal.Platform_commonKt.serializerNotRegistered(Platform.common.kt:90)
	at kotlinx.serialization.SerializersKt__SerializersKt.serializer(Serializers.kt:299)
	at kotlinx.serialization.SerializersKt.serializer(Unknown Source)
	at androidx.navigation.NavDestinationBuilder.<init>(NavDestinationBuilder.kt:91)
        ...

When I place the same class in the test source set, the serializer is available in my test.

To Reproduce

# gradle.properties
android.experimental.enableTestFixtures=true
android.experimental.enableTestFixturesKotlinSupport=true

// project level build.gradle.kts
plugins {
    alias(libs.plugins.kotlin.serialization)
}

dependencies {
    testImplementation(libs.kotlinx.serialization.json)
    testFixturesImplementation(libs.kotlinx.serialization.json)
}

Expected behavior

When I move the @Serializable class from the test source set to testFixtures, the serializer is still available in my test.

Environment

• Kotlin version: 2.1.10
• Library version: 1.8.0
• Kotlin platforms: JVM
• Gradle version: 8.10.2
• IDE version: N/A